Commercially produced aluminum microport tubing for use in brazed applications is generally produced in the following manner. The extrusion ingot is cast and optionally homogenized by heating the metal to an elevated temperature and then cooling in a controlled manner. The ingot is then reheated and extruded into microport tubing. This is generally thermally sprayed with zinc before quenching, drying and coiling. The coils are then unwound, straightened and cut to length. The tubes obtained are then stacked with corrugated fins clad with filler metal between each tube and the ends are then inserted into headers. The assemblies are then banded, fluxed and dried.
The assemblies can be exposed to a braze cycle in batch or tunnel furnaces. Generally, most condensers are produced in tunnel furnaces. The assemblies are placed on conveyor belts or in trays that progress through the various sections of the furnace until they reach the brazing zone. Brazing is carried out in a nitrogen atmosphere. The heating rate of the assemblies depends on the size and mass of the unit but the heating rate is usually close to 20° C./min. The time and temperature of the brazing cycle depends on the part configuration but is usually carried out between 595 and 610° C. for 1 to 30 minutes.
A difficulty with the use of aluminum alloy products in corrosive environments, such as automotive heat exchanger tubing, is pitting corrosion. Once small pits start to form, corrosion actively concentrates in the region of the pits, so that perforation and failure of the alloy occurs much more rapidly than it would if the corrosion were more general. With such a large cathode/anode area ratio, the dissolution rate at the active sites is very rapid and tubes manufactured from conventional alloys can perforate rapidly, for example in 2-6 days in the SWAAT test.
Zinc coating applied to the tube after extrusion acts to inhibit corrosion of the tube itself. However during the braze cycle, the Zn layer on the extruded tube starts to melt at around 450° C. and once molten, is drawn into the fillet/tube joint through capillary action. This occurs before the Al—Si cladding (fin material) melts at approximately 570° C. and as result the tube-to-fin fillet becomes enriched with Zn, rendering it electrochemically sacrificial to the surrounding fin and tube material. A problem with thermally spraying with zinc before brazing is therefore that the braze fillets become zinc enriched and tend to be the first parts of the units to corrode. As a result, the fins become detached from the tubes, reducing the thermal efficiency of the heat exchanger. In addition to these physical effects, any enrichment of the fillet region with Zn has the effect of reducing the thermal conductivity of the prime heat transfer interface between the tube/fin. There is also a desire to move away from the use of zinc for cost savings and for workplace environment reasons.
In an assembly of brazed tubes and fins, it has been found to be advantageous to have the fins corrode first and thereby galvanically protect the tubes. Most fin alloys used with extruded tubes are clad alloys where the core alloys are either 3XXX or 7XXX series alloy based and contain some zinc to make them electronegative, and thereby provide this type of protection. By making the fin sufficiently electronegative, the tubes to which the fins are brazed can be protected, in this way, if the zinc content of the fin is raised sufficiently. However, this has a negative impact on the thermal conductivity of the fin and on the ultimate recyclability of the unit. Furthermore, if the fin material is too electronegative it can corrode too fast and thereby compromises the thermal performance of the entire heat exchanger. Corrosion potential and the difference between corrosion potential of tube and fin have been frequently used to select tube and fin alloys to be galvanically compatible (so that the fin corrodes before the tube). This technique serves to give an approximate galvanic ranking. In order to obtain a true determination of the performance of such combinations it has been found that a measurement of the direction and magnitude of the galvanic current permits a better determination of ultimate performance. Little attempt has been made to optimize the tube-fin combination in heat exchangers based on extruded tubes through the use of appropriate alloys alone, the use of zinc cladding being widely used instead. One constraint on such optimization is that it still also must be possible to extrude the tubes without difficulty.
Anthony et al., U.S. Pat. No. 3,878,871, issued Apr. 22, 1975, describes a corrosion resistant aluminum alloy composite material comprising an aluminum alloy core containing from 0.1 to 0.8% manganese and from 0.05 to 0.5% silicon, and a layer of cladding material which is an aluminum alloy containing 0.8 to 1.2% manganese and 0.1 to 0.4% zinc.
Sircar, U.S. Pat. No. 5,785,776, issued Jul. 28, 1998, describes a corrosion resistant AA3000 series aluminum alloy containing controlled amounts of copper, zinc and titanium. It has a titanium content of 0.03 to 0.30%, but this level of titanium raises the pressures required for extrusion, which will ultimately lower productivity.
In Jeffrey et al., U.S. Pat. No. 6,284,386, issued Sep. 4, 2001, extruded aluminum alloy products having a high resistance to pitting corrosion are described in which the alloy contains about 0.001 to 0.3% zinc and about 0.001 to 0.03% titanium. The alloys preferably also contain about 0.001 to 0.5% manganese and about 0.03 to 0.4% silicon. These extruded products are particularly useful in the form of extruded tubes for mechanically assembled heat exchangers.
It is an object of the present invention to provide brazed extruded aluminum alloy tubing for heat exchangers having adequate corrosion resistance without special treatments, such as thermal spraying of the surface with zinc, and also being galvanically compatible with fins joined thereto.
It is a further object of the present invention to provide a brazed heat exchanger assembly consisting of extruded tubing and fins in which the tubing alloy is optimized to minimize self corrosion and so that the heat exchanger is protected from overall corrosion by a slow corrosion of the fins.